Yunalesca's first form shouldn't be a problem for you but basically just keep your health up.

The second form is a little tricky but with a little pre-battle knowledge, you can get past her. Don't remedy Zombie when she inflicts you with it. Instead, leave your characters in Zombie status. Rely on switching characters out instead of healing. Basically, just keep fighting with the same Zombied party until you health is dangerously low. Switch characters out instead of wasting turns to heal Zombied characters.

Make sure you have a Zombied party when she switches into her third and final form because she'll use Mega Death. 100% chance of killing NON-zombied characters.

Now, to make the second and third forms easy, I used Overdrives. Charge up all characters and aeons overdrives before the battle. Use the character drives on the second form and let the aeons rip for the third form. Should be much much easier this way.